As is well known, the evaporator in an air conditioning system functions with other parts of the system to take heat out of incoming air and provide cool, dehumidified air delivery. For example, in a motor vehicle air conditioning system, refrigerant is typically circulated through the components of the system picking up heat from the incoming passenger compartment air at low pressure in the evaporator and giving off heat outside the passenger compartment at the condenser at high pressure. Evaporator cores are made in various types of materials and construction. One common type of construction for motor vehicle use is called a "plate and fin", so named for the plates that form the fluid passes in the exchanger. Another type of construction is called a "serpentine", so named for the winding shape of its tube(s). It is the latter which is currently receiving renewed interest for use in the motor vehicle industry.
In the typical serpentine tube heat exchanger, there is a single flat sided tubular aluminum extrusion that is bent into the serpentine shape with fittings then provided on the ends for refrigerant inlet and outlet flow. Corrugated fins are inserted between adjacent straight sections of the tubes for heat conduction from the air to the refrigerant with the fin height set by the minimum bend radius of the tubular extrusion.
In such serpentine tube evaporators, the heat transfer is limited mainly due to a high refrigerant side resistance to flow (pressure drop) due to the length of the flow path being the length of the serpentine tube. The pressure drop causes the average refrigerant saturation pressure and temperature to be higher than in an evaporator where the average pressure is closer to the outlet pressure. In a motor vehicle air conditioning system, the evaporator outlet pressure is the controlling factor and an evaporator where the average core pressure is closer to the outlet pressure will have a lower refrigerant temperature and hence better performance. Furthermore, the air flow entering a typical motor vehicle evaporator is not uniformly distributed across the core face due to the tortuous air path in the heating, ventilating and air conditioning system. And this results in an unbalanced heat transfer rate where split circuits across the core face are employed as one circuit will have insufficient liquid refrigerant and cause reduced overall performance.
The conventional serpentine tube evaporator also typically suffers from low performance due to the lack of mixing of the refrigerant in the tubular passages as it flows through the core. Evaporative heat transfer is more efficient than sensible heat transfer and the refrigerant entering the core near the edge of the extruded tube nearest the hot entering air tends to vaporize sooner than refrigerant flowing near the cool air exiting edge. Thus, there is vapor flowing through the core at the front edge without any source of refrigerant liquid available for additional evaporation. Likewise, at the rear edge where the liquid is flowing, there is insufficient heat coming into the tube at that point to promote evaporation. Movement of the liquid refrigerant to the hotter tube areas to the cooler areas is not normally possible because of structural webs normally formed in the extruded tube. Furthermore, where there are split circuits across the core face, there may be unevaporated liquid at the split that is not made uniformly available to the different downstream tube paths. And this early maldistribution of liquid refrigerant in the flow path can cause a substantial loss in performance of an entire half of the core with maldistributed air flow.